Field of the Invention
The present invention concerns magnetic resonance (MR) imaging, and in particular concerns simultaneous multi-slice (SMS) MR imaging.
Description of the Prior Art
MR imaging is a widely used imaging modality for medical diagnosis as well as for material inspection.
In a magnetic resonance apparatus, the examination object (a patient, in the case of medical magnetic resonance imaging) is exposed to a strong and constant basic magnetic field, by the operation of a basic field magnet of an MR scanner, in which the examination object is situated. The MR scanner also has a gradient coil arrangement that is operated in order to activate gradient fields that spatially encode the magnetic resonance signals. The magnetic resonance signals are produced by the radiation of radio-frequency (RF) pulses from an RF radiator, such as one or more antennas, in the MR scanner. These RF pulses excite nuclear spins in the examination object, and are therefore often called excitation pulses. The excitation of the nuclear spins at an appropriate frequency gives the excited spins a magnetization that causes the nuclear spins to deviate, by an amount called the flip angle, from the alignment of the nuclear spins that was produced by the basic magnetic field. As the nuclear spins relax, while returning to alignment in the basic magnetic field, they emit MR signals (which are also RF signals), which are received by suitable RF reception antennas in the MR scanner, which may be the same or different from the RF radiator used to emit the excitation pulse.
The emitted MR signals have a signal intensity that is dependent on the exponential decay over time of the magnetization of the nuclear spins. The acquired signals are digitized so as to form raw data, which are entered into a memory that is organized as k-space, as k-space data. Many techniques are known for reconstructing an image of the examination object from the k-space data.
By appropriately selecting different characteristics of the MR data acquisition sequence that is used, the acquired signals can be differently weighted so that different sources of the detected MR signals (i.e., different tissues in the case of medical MR imaging) appear with different contrasts in the reconstructed image. In the case of medical MR imaging, a weighting is selected that causes the tissue that is important for making the intended medical diagnosis to have the best contrast (brightness) in the reconstructed image. One such type of weighting is known as T1-weighting, because it depends on the so-called T1 relaxation time of the nuclear spins.
Many different techniques are known for acquiring the raw MR data. One such technique is known as simultaneous multi-slice (SMS) acquisition, which is a technique for accelerating the acquisition of the data from a given volume of the examination object, wherein nuclear spins in multiple slices are excited simultaneously, and the resulting MR signals are simultaneously acquired from each slice. This results in a dataset in k-space that is composed of data from the multiple slices collapsed on top of each other. Techniques are known for separating or uncollapsing the data for these respective slices during image reconstruction, such as the slice GRAPPA (Generalized Autocalibration Partially Parallel Acquisitions) technique, which is schematically illustrated in FIG. 1. In the example shown in FIG. 1, multiple slices S1, S2 and S3 are excited simultaneously, resulting in each slice generating an echo train of magnetic resonance signals, which are acquired according to the known blipped CAIPIRINHA (Controlled Aliasing in Parallel Imaging Results in Higher Acceleration) technique.
Excitation of the nuclear spins in the simultaneously acquired slices is implemented with a multi-band (MB) RF pulse. An MB RF pulse is generated by the superimposition of a number of individual single band (SB) RF pulses, of the type that are typically used to excite nuclear spins in a single selected slice in conventional magnetic resonance imaging.
As noted above, the received or detected signals that result from the excitation of the nuclear spins can be given a weighting so that the signal intensity is dependent on the T1 relaxation time of the excited nuclear spins. T1 mapping (which differs from the reconstruction of a T1-weighted image) is the quantifying of the T1 relaxation time of the tissue from which the signals originate by, pixel-by-pixel in a 2D image, or voxel-by-voxel in a 3D image, analysis of the signal intensities in the reconstructed MR image. The result of this analysis is a representation of the spatial distribution of the per pixel, or per voxel, T1 values, i.e., a T1 map. T1 mapping using a multi-flip angle gradient echo sequence (i.e., a conventional single slice sequence) is implemented in commercially available magnetic resonance systems from Siemens Healthcare. This sequence requires several scans to be executed, respectively with different flip angles. A voxel-by-voxel fit of the respective images is then performed, in order to calculate a T1 map.
Conceptually, SMS might be considered as a possible technique for accelerating the acquisition of raw MR data for generating such T1 maps. Simply applying SMS acceleration to the generation of T1 maps, however, presents problems that must be taken into account and overcome, in order to make SMS acceleration meaningful in clinical practice for acquiring raw data for the subsequent generation (calculation) of T1 maps.
As shown in FIG. 2, in a conventional SMS sequence, the MB RF pulse that is radiated in an SMS sequence is formed by the simultaneous emission and superimposition of a number of SB RF pulses, that all have the same flip angle. (Although these respective pulses may have different phase curves, as shown in FIG. 2, the respective phases of the SB RF pulses, and the resulting MB RF pulse, are not relevant to the discussion herein.)
Simply applying a conventional SMS sequence in a multi-flip angle T1 mapping protocol would mean executing multiple scans with progressively increasing flip angles. For example, for acquiring data from two slices, an SMS acceleration factor of two, and two flip angles, namely Flip1 and Flip2, with Flip2>Flip1, the following two scans would have to be executed:Scan1 using MBPulse1=SBPulse_Slice1_Flip1+SBPulse_Slice2_Flip1Scan2 using MBPulse2=SBPulse_Slice1_Flip2+SBPulse_Slice2_Flip2
In this case, Scan2 (using MBPulse2) has a higher RF energy compared to Scan1 (using MBPulse 1), and may produce problems if the specific absorption rate (SAR) limit for the patient is exceeded.
Additionally, Scan2 (using MBPulse2) has higher peak RF power requirements compared to Scan1 (using MBPulse1), and may encounter problems if the peak RF requirement is too high.
Another type of mapping that is commonly employed in MR imaging is called B1 mapping. The B1 field is the RF field that is collectively produced in the examination object by the RF radiator. In the case of a non-uniform examination object such as a patient, different tissue and other objects such implants have respectively different magnetic susceptibilities and therefore differently affect the RF field within the patient. As a result, even though the RF radiator may be designed to emit a spatially uniform RF field, the RF field that actually occurs within the patient will have a non-uniform strength (magnitude) distribution. Techniques are known for making measurements that detect the different strengths of the RF field at a multitude of different spatial locations within the patient so that a map of this spatial distribution can then be calculated. This is known as B1 mapping. Such a B1 map has many uses in the context of MR imaging, such as correcting the resulting MR image, or making adjustments in the positions of the RF radiator and/or the patient prior to acquiring the diagnostic MR data.